A number greater than 0 that specifies what weight to give the master when choosing which backend to execute a read query on.
A value of 0,
which is the default,
does no reads from master,
while a value of 1 gives it the same priority as any single replicant.

For example: if you have 2 replicants,
and a "master_read_weight" of 0.5,
the chance of reading from master will be 20%.

You can set it to a value higher than 1,
making master have higher weight than any single replicant,
if for example you have a very powerful master.